1. Field of the Invention
This invention relates, in general, to photocopying and, more specifically, to drive systems for document scanning apparatus.
2. Description of the Prior Art
Document scanners are used for scanning or reading the image content of an original document. Sometimes, the scanner is used to optically expose lines of the document onto another surface. Electronic scanners, which have means for converting the light from the document into electrical signals, are becoming popular for use with electronic copiers and computers. Regardless of the use and the equipment to which it is connected, the scanner must contain accurate and reliable mechanical devices for moving the scanner across the document, or for moving the document across the scanner.
One type of scanner which is in wide use uses a moving carriage assembly which is positioned underneath the document and moves across the complete length of the document. The carriage assembly contains some or all of the optical and electronic or reflective components needed to move across the document during a scan. Drive mechanisms for such devices are varied, but one or more cables are frequently used to move the carriage. These cables are driven by pulleys, shafts, drums, etc., over which the cables are wound.
The arrangement of the cables around the pulleys and drive members usually requires certain restrictions on the size, placement, and complexity of the scanning apparatus. This frequently is necessitated by the fact that the required winding of the cables on the drive member cause the cables to enter and exit the drive member at different axial locations. This makes the cables not aligned with each other and any return pulley in the system needs to be oriented at an angle to accommodate the "tilted" cable orientation. This requirement adds to the complexity of the drive mechanism and to the overall size of the scanner.
U.S. Pat. No. 4,710,017, issued on Dec. 1, 1987, discloses a scanner suitable for use with a copy machine wherein the cable arrangement requires obliquely oriented pulleys in the system. While not identified with reference numbers, FIG. 10 of that patent shows the drive cables wound around cylindrical drive members with the two ends of the cable leaving the drive member at different axial positions. Thus, a nearby pulley, which is oriented obliquely to the drive member, is needed to move one end of the cable over in alignment with the other end of the cable leaving the drive member.
U.S. Pat. No. 4,771,315, issued on Sept. 13, 1988, discloses another arrangement known in the prior art which compensates for the offset of the cables. In FIG. 2 of that patent, the main drive pulley 14 is separated from the driving member or pulley 15. The cables around the pulley 15 appear to be in substantial alignment only because they do not wind around the pulley 15 for several turns, as they do around the drive pulley 14.
Therefore, it is desirable, and it is an object of this invention, to provide a scanner drive mechanism which eliminates the need to have obliquely aligned pulleys on the system, cables not aligned over one another, or extra drive components to compensate and correct for cable offset.